This invention relates to a powder coating system in which a mixture of an epoxy resin and a low temperature curing agent is extruded as one component and pulverized to form a low temperature curable coating powder. The powder cures at a temperature of from about 225° F. to about 300° F. and produces a coating having an exceptionally smooth surface with either a low or high gloss. This invention also relates to the electrostatic coating of massive metal articles and wood and low temperature curing to achieve a pleasing smoothness. It also relates to a method for compounding a coating powder by extruding a mixture comprising an epoxy resin having a low melt viscosity and a low temperature curing agent at a temperature of from about 160° F. to about 220° F. (about 71-104° C.).
Traditionally, coating powders have been made by the extrusion of a mixture of resins and curing agents to obtain a homogeneous mixture and then grinding the extrudate and screening the comminuted product to obtain the desired particle sizes and particle size distribution. The powder is then electrostatically sprayed onto a substrate, traditionally a metal substrate, and cured at temperatures much higher than 200° F. Achieving a powder coating composition that will cure on heat sensitive substrates at less than 300° F. in less than 5 minutes has long been a goal of the industry. The curing of powder coatings on materials such as wood, plastic, and the like has been limited by the fact that the extrusion of a mixture of a resin and a low temperature curing agent, i.e., one that is active at 250° F. or less, would cause the coating powder to gel in the extruder because the extrusion typically generates enough heat to raise the temperature to 200° F. or higher.
This problem has been avoided by extruding the resin and a small amount of catalyst or low temperature curing agent, about 0.1 to 8 phr (per hundred parts resin), which is insufficient to cause substantial curing of the resin during extrusion, grinding the extrudate, and then blending the resultant powder with an additional amount of the curing agent in powder form to form a coating powder.
The high temperatures that are generated by the extrusion of a curing agent with a resin are caused in part by the fact that the temperature must be sufficient to facilitate the mixing of resins having initially high melt viscosities. Even higher temperatures are produced by the friction arising from the mixing of the still highly viscous molten resins with curing agents that are solid at room temperature.
Douglas S. Richart said in his article published in the April, 1996 issue of POWDER COATINGS, that the coating of wood with a low temperature cure powder is next to impossible because the coating must be cured at a temperature below 200° F. and the resin must have a flow temperature of about 10 to 20 degrees lower than that. Resins having lower melt viscosities, however, generally have lower glass transition temperatures. The presence of a resin having a very low Tg in the powdered extrudate increases the tendency to sinter and thus decreases the blocking resistance of the powder during storage at the normal maximum temperature of about 80° F. (27°C.).